VARIABILITY AMONG BACTERIA 25 



convert these nitrites into nitrates. Both can derive their necessary 

 carbon from alkaline carbonates. Other bacteria, or organisms allied to 

 the bacteria, exist which can actually take up and combine into new- 

 compounds the free nitrogen of the air. These are found in the tubercles 

 •which develop on the rootlets of the leguminosae. Without such 

 organisms the tubercles do not' develop, and without the development of 

 the tubercles the plants are poor and stunted. Bacteria thus play an 

 important part in the enrichment and fertilisation of the soil. 



The Occurrence of Variability among Bacteria. — The question of the 

 division of the group of bacteria into definite species has given rise to 

 much discussion among vegetable and animal morphologists, and at one ' 

 time very divergent views were held. Some even thought that the 

 same species might at one time give rise to one disease, — at another time 

 to another. There is, however, now practical unanimity that bacteria 

 show as distinct species as the other lower plants and animals, though, 

 of course, the difficulty of defining the concept of a species is as great in 

 them as it is in the latter. Still, we can say that among the bacteria we 

 see exhibited (to use the words of De Bary) "the same periodically 

 repeated course of development within certain empirically determined 

 limits of variation" which justifies, among higher forms of life, a species 

 to be recognised. What at first raised doubts as to the occurrence of 

 species among the bacteria was the observation in certain cases of what is 

 known as pleomorphism. By this is meant that one species may assume 

 at different times different forms, e.g., appear as a coccus, a bacillus, or 

 a leptothrix. This is especially the case with certain bacilli, and it may 

 lead to such forms being classed among the higher bacteria. Pleomor- 

 phism is, however, a rare condition, and with regard to the bacteria as a 

 whole we may say that each variety tends to conform to a definite type of 

 structure and function which is peculiar to it and to it alone. On the 

 other hand, slight variations from such type can occur in each. The size 

 may vary a little with the medium in which the organism is growing, and 

 tfnder certain similar conditions the adhesion of bacteria to each other may 

 also vary. Thus cocci, which are ordinarily seen in short chains, may grow 

 in long chains. The capacity to form spores may be altered, and such 

 properties as the elaboration of certain ferments <or of certain pigments 

 may be impaired. Also the characters of the growths on various media 

 may undergo variations. As has been remarked, variation as observed 

 consists largely in a tendency in a bacterium to lose properties ordinarily 

 possessed, and all attempts to transform one bacterium into an apparently 

 closely allied variety (such as the b. cqli into the b. typhosus) have 

 failed. This of course does not preclude the possibility of one species 

 having been originally derived from another, or of both having descended 

 from a common ancestor, but we can say that only variations of an 

 unimportant order have been observed to take place, and here it must 

 be remembered that in many cases we can have forty-eight or more 

 generations under observation within twenty-four hours. 



